In 2008, per Wikipedia, the national popular vote for the House went 53% for the Democrats and 43% for the Republicans. That translates to a 55 D-45 R two party vote when we subtract other votes. That gives the Democrats a 10 point advantage in 2008.

The current Gallup Generic Ballot reads 43 for the Democrats and 49 for the Republicans. That translates to a two party vote of 47 D-53 R, or a Republican advantage of 6.

From R-10 to R+6 adds up to a 16 point swing from 2008. If every district moved 16 points from 2008, the Swingometer projects a 45 seat Republican gain, for 223 seats in the House and the majority.

This will typically be a low end projection in a wave year, additionally, because the “national climate” influences fundraising, voter enthusiasm, retirements, and recruitment.